1. Field of the Invention
This invention relates to a wavelength multiplex light source and a wavelength control method for use in wavelength multiplex communication or the like.
2. Related Background Art
As a wavelength change-over apparatus, there is known one as described in Japanese Laid-Open Patent Application No. 5-190958 wherein a plurality of light sources are used and by an optical switch, a light output may be taken out of one of the light sources to thereby effect wavelength change-over.
FIG. 1 of the accompanying drawings shows a diagram for illustrating an example of the prior art. Reference numerals 101 and 102 designate light sources, reference numeral 103 denotes an optical switch for selecting and outputting light from one light source 101 or 102, reference numeral 104 designates a modulator for modulating the light from the optical switch 103 in accordance with a signal, reference numerals 105 and 106 denote wavelength stabilizing devices, and reference numeral 107 designate a control device for controlling the wavelength stabilizing devices 105, 106 and the optical switch 103. The light sources 101 and 102 have their wavelengths set and stabilized by the wavelength stabilizing devices 105 and 106. In such a construction, one light source 101 or 102 has been selected by the optical switch and used and in the meantime, the setting and stabilization of the wavelength of the other light source 102 or 101 could be effected.
In the above-described example of the prior art, however, one light source is required for one wavelength and it is necessary to use two light sources for the change-over of two wavelengths.
Also, as light sources used for wavelength division multiplexing transmission, there are known a so-called distributed feedback type semiconductor lasers (DFB-LD) in which an oscillation spectrum is stable and the expanse (line width) of the spectrum during modulation is small so that it can be made highly densely multiplex. In these DFB-LDS, the structure of an oscillator is made into a multielectrode and the gain and phase can be controlled to make the transmitted wavelength variable. By making the most of this variable wavelength characteristic, the channels of multiplex wavelengths which have heretofore been fixed can be freely selected and more flexible optical transmission can be realized. Therefore, this becomes effective for multiplex wavelength transmission for optical LAN in which high speed video signals are mutually exchanged.
In the above-described example of the prior art, however, the wavelength variation width is limited to the order of 1 nm and in the direct modulation of DFB-LD, the expanse of the spectrum during modulation is of the order of 0.2-0.3 nm as compared with a Fabry-Perot type laser, and this has led to the problem that the substantial number of multiplex wavelengths is limited to three to four wavelengths. Besides DFB-LD, an attempt to make the grating period non-uniform and make the wavelength variation width into a wide band has also been reported, but this has suffered from the problem that the element becomes bulky and the structure becomes complicated. Also, as a construction for suppressing the expanse of the spectrum during modulation, there have been reported several examples of the trial manufacture of an optical integrated device in which modulators are integrated in a continuously oscillated DFB-LD, but these have suffered from the disadvantage that a high degree of integration process is required and the yield becomes poor.